Removing a layer in a sample such as a semiconductor die involves removing very small amounts and very thin layers of an integrated circuit, which contains metals and dielectrics, to reveal the underlying circuitry in a precise and controlled manner. Typical methods include wet chemical etching, dry (plasma) etching, and mechanical polishing or physical abrasion.
Mechanical polishing is performed by manually polishing the sample using polishing pads and abrasive slurries to erode the surface of the sample to the extent needed. The problem faced during this process is the uneven erosion of the periphery and surface, wherein for example, copper is removed slower than SiO2. This leads to the non-uniform removal of a given surface, due to various levels of stress exerted in different spots, or variations in feature density of the sample during labouring the sample.
Wet (chemical) etching is performed by using chemicals and immersing a sample into the chemical to cause a chemical reaction to remove material from the sample surface. This is very difficult to control as the rates at which the chemicals etch the various materials in the sample vary, and material interfaces can be severely affected, which once again leads to the non-uniform removal of materials.
Dry (plasma) etching is performed by using combinations of non-reactive gasses and/or reactive gasses, ionized under vacuum in a strong electric field. Reactive ions cause both chemical reactions on a sample and physical bombardment, thereby removing material from the sample, whereas non-reactive ions cause only physical bombardment of the sample and thereby eroding (knocking-off) the sample. The non-uniformities in material density and etch species concentration adversely affect the etch rate and subsequent removal processes.
Ion beam milling is also used for material removal in samples by etching or milling a sample. Ion beam mills may be used for various other purposes in the semiconductor industry, such as film deposition or surface modification or activation. Using an ion beam source with both reactive and non-reactive gases, the source gas is ionized and the positive ions are extracted and accelerated toward the sample residing on a rotatable cooled stage in a vacuum chamber. The angle of the sample stage can be adjusted for the desired impact of the ions on the surface of the sample. There are various Ion Milling systems known in the art, such as Focussed Ion Beam Milling (FIB) systems and Broad Ion Beam Milling (BIB) systems.
Very narrow (small diameter) ion beams, typically with gallium ions, are used in FIB systems to remove material in precise locations in a sample (often on semiconductor integrated circuits) and also to deposit new materials on the ICs. This is used to edit the circuits, rerouting connections to repair damage or introduce new functionality. FIB systems are also used to cross section samples, build novel physical structures, and physically shape material (micromachining) on a very small scale. A typical area shaped by the ion beam would be measured in microns, or at most, tens of microns. The sample is kept stationary, while the ion beam is scanned back and forth. Beam to sample angle can be controlled by tilting the sample. The target areas capable of being practically modified by FIB are restricted to small, due to the relatively slow milling rate of FIB systems. In addition, there are a number of other aspects relating to a small scanned beam that make it quite difficult to accurately modify large areas, including dwell time, overlap area, proximity between scans and features, that are all exacerbated as, for example, the very narrow beam is passed over the entire surface of a sample (such as an integrated circuit).
Medium diameter ion beams (millimeter sized) are typically used to ‘clean up’ a sample, removing surface damage generated in previous steps. One example is during transmission electron microscopy (TEM) sample preparation; a sample is polished using physical abrasives until it is very thin, then a medium diameter ion beam (often using Argon ions) is used to abrade the surface and gently mill away a thin layer (of nanometers thickness). The beam is kept stationary while the sample is typically rotated or scanned back and forth, or both. Beam to sample angle is usually adjustable by moving the ion gun. Milled area is measured in hundreds of microns, or in millimeters.
Finally, broad ion beam milling systems (centimeters in diameter) are also used in the fabrication process of semiconductor devices. A layer of a sample is masked, when the sample is exposed to the beam, material is removed over a large area where not protected by the mask. The gun is stationary but the sample can be rotated and tilted to different angles. Milled area is measured in centimeters. The material removed is typically homogenous in nature (a layer of a single material or single compound is milled until removed). BIB mills have been limited to removing a layer of homogenous material as the removal rate is maintained constant for a given homogenous layer until the next layer is reached. BIBmilling ion guns are associated with “grids” or “fields” in front of the ion gun that are capable of changing parameters of the beam. Typical beam spreads in broad beam ion gun applications are in the range of 5 to 20 cm. Typically, broad ion beam applications in Integrated Circuits (IC) include deposition and de-layering when building structures on an IC.
In deposition applications, broad ion beams are directed at a material source. The ion beam bombards the material source and causes the atoms of the material source to be ejected therefrom. A substrate is placed in a location where the ejected material source will hit and bond as a layer thereto in a more or less even fashion. The substrate can be moved linearly (in x, y and z directions) and rotated (about x, y and z axes—which would include a change in tilt angle of the substrate, relative to the main [?] direction of impact of the ejected material source). A mask can be used to create pre-defined structures on the substrate. Alternatively, material can be deposited on the mask beforehand in a predefined pattern that, when removed, causes the deposited material to remain on the substrate in a negative image of the predefined pattern.
In material removal applications, broad ion beams are directed at a sample in order to remove sample material in a non-selective manner. Generally, when a mask is pre-applied to the sample or a masking material is deposited on the sample beforehand in a predefined pattern. Known systems are directed to unselectively remove homogenous material layers of the sample without eroding the mask or the sample under the mask to facilitate creation of structures on an IC. The angle of the sample may be adjusted to maximize the removal rates for a substantially homogenous material layer. An endpoint detection system may also be used to detect when the substantially homogenous material layer has been substantially removed and the material from a subsequent layer is being removed, at which point removal is stopped.
U.S. patent application Ser. No. 11/205,522, discloses a “Method of Reworking Structures Incorporating Low-K Dielectric Materials”. U.S. patent application Ser. No. 11/661,201 discloses “Directed Mult-Deflected Ion Beam Milling of a work Piece and Determining and Controlling Extent Thereof”. Further, U.S. Pat. No. 5,926,688 discloses “Method of Removing Thin Film Layers of a Semiconductor Component”. However, none of the noted patent or patents overcomes the shortcomings in the general area of delayering a sample.
Therefore there is a need for a method and system to overcome some of the shortcomings in the general area of delayering a sample.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present technology. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.